1. Field
The present invention relates generally to data communication, and more specifically to techniques for performing equalization of multiple signals received for soft handoff in wireless communication systems.
2. Background
Wireless communication systems are widely deployed to provide various types of communication such as voice, packet data, and so on. These systems may be multiple-access systems capable of supporting communication with multiple users and may be based on code division multiple access (CDMA), time division multiple access (TDMA), frequency division multiple access (FDMA), or some other multiple access technique. These systems may also be wireless local area network (LAN) systems, such as those that implement the 802.11b IEEE standard.
A wireless terminal in a CDMA system may receive data transmission from one or more base stations on the forward link. If the terminal is in soft handoff, then the same data stream may be transmitted to the terminal by multiple base stations to provide diversity against deleterious path effects and improve reliability. The signal transmitted by each base station may reach the terminal via one or more propagation paths. The received signal at the terminal may thus include one or more signal instances (or multipath components) of the signal transmitted by each base station.
A terminal in a CDMA system typically employs a rake receiver to process the received signal. The rake receiver normally includes one or more searcher elements and a number of demodulation elements, which are commonly referred to as “searcher” and “fingers”, respectively. Due to the relatively wide bandwidth of a CDMA waveform, the communication channel may be assumed to be composed of a finite number of resolvable multipath components. Each multipath component may be characterized by a particular time delay and a particular complex gain. The searcher then searches for strong multipath components in the received signal, and fingers are assigned to the strongest multipath components found by the searcher. Each finger processes its assigned multipath component to provide symbol estimates for that multipath component. The symbol estimates from all assigned fingers are then combined to provide combined symbol estimates for the data stream to be recovered.
The rake receiver can provide acceptable performance for a CDMA system operated at a low signal-to-noise-and-interference ratio (SINR). The rake receiver can combine energy from a number of multipath components received by the terminal for one or multiple signals transmitted by one or multiple base stations. However, the rake receiver generally does not remove the effects of inter-symbol interference (ISI) that results from multipath effects and other distortions in the wireless channel. This ISI is typically negligible at low SINRs but can become a non-negligible noise component at higher SINRs. The ISI may limit the SINR that may be achieved by the terminal and may thus have a large impact on performance.
There is therefore a need in the art for techniques to process a received signal while in soft handoff to provide combined symbol estimates having higher SINR.